<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    Array.prototype.groupBy = function (fn) {
      const res = {}
      for (const item of this) {
        const key = fn(item);
        if (!res.hasOwnProperty(key)) res[key] = [];
        res[key].push(item)
      }
      return res
    };
    array = [
      { "id": "1" },
      { "id": "1" },
      { "id": "2" }
    ],
      fn = function (item) {
        return item.id;
      }
    console.log(array.groupBy(fn)); 
  </script>
</body>

</html>